This invention relates to optical communication systems, and more particularly to add-drop optical switches and methods of fabricating same.
Optical communication systems are increasingly being used to communicate data, voice, multimedia and/or other communications. Optical communication systems may employ optical fibers and/or free space optical communication paths. It will be understood by those having skill in the art that optical communication systems may use optical radiation in the visible, ultraviolet, infrared and/or other portions of the electromagnetic radiation spectrum.
An important component in optical communications is the add-drop optical switch, also referred to as an add-drop multiplexer. As is well known to those having skill in the art, an add-drop multiplexer receives optical radiation from an IN optical path and transmits this optical radiation to an OUT optical path. However, the add-drop optical switch also has the capability of removing an optical signal from the IN optical path and placing the signal on a DROP optical path. The add-drop optical switch also has the capability to place an optical signal on an ADD optical path, so that the optical signal from the ADD optical path is placed on the OUT optical path. Accordingly, the add-drop optical switch can selectively couple the IN optical path to the OUT optical path, the IN optical path to the DROP optical path and the ADD optical path to the OUT optical path. Add-drop optical switches can employ an array of fixed and/or movable reflectors, such as mirrors, to perform the above-described selective coupling. Add-drop optical switches are described, for example, in U.S. Pat. Nos. 5,778,118; 5,960,133 and 5,974,207, and need not be described further herein.
It has been proposed to fabricate add-drop optical switches using microelectromechanical system (MEMS) technology. As is well known to those having skill in the art, MEMS devices are potentially low cost devices, due to the use of microelectronic fabrication techniques. New functionality also may be provided, because MEMS devices can be much smaller than conventional electromechanical devices.
Unfortunately, it may be difficult to fabricate add-drop optical switches using MEMS technology. In particular, it may be difficult to fabricate reflectors that are oriented orthogonal to one another using MEMS fabrication processes. This potential difficulty now will be described in connection with FIG. 1.
Referring now to FIG. 1, a conventional MEMS add-drop optical switch 100 is shown. As shown in FIG. 1, a conventional MEMS add-drop optical switch 100 can include a substrate 110, generally a monocrystalline silicon substrate. An IN optical path 120 on the substrate receives optical radiation. An OUT optical path 130 on the substrate transmits optical radiation. An ADD optical path 140 on the substrate receives optical radiation and a DROP optical path 150 on the substrate transmits optical radiation. The ADD, IN, OUT and DROP optical paths 140, 120, 130 and 150 all are oriented on the substrate 110 in parallel, on opposite sides of the substrate 110. A first fixed mirror 180 and a second fixed mirror 190 are fixedly coupled to the substrate 110. A first movable mirror 160 and a second movable mirror 170 are movably coupled to the substrate 110 for movement to and away from a radiation reflecting position as shown by the respective arrows 162 and 164. The fixed mirrors 180 and 190 and the movable mirrors 160 and 170 are arranged on the substrate 110, to selectively couple the IN optical path 120 to the OUT optical path 130, to selectively couple the IN optical path to the DROP optical path 150 and to selectively couple the ADD optical path 140 to the OUT optical path 130.
As shown in FIG. 1, the adjacent fixed mirrors 180 and 190 and movable mirrors 160 and 170 are oriented orthogonal (at a 90xc2x0 angle) to one another. Unfortunately, it may be difficult to fabricate orthogonally oriented mirrors on a monocrystalline silicon substrate 110. In particular, since monocrystalline silicon does not include orthogonal crystalline planes, it may be difficult to fabricate orthogonal mirrors using conventional wet etching methods. Reactive Ion Etching (RIE) can be used to make the configuration shown in FIG. 1. Unfortunately, reactive ion etching may produce surface imperfections that can degrade the quality of the mirrors, so that the add-drop optical switch 100 may have degraded performance compared to that obtained by wet etching along the crystalline planes.
The present invention can provide add-drop optical switches that include fixed reflectors, such as fixed mirrors, and movable reflectors, such as movable mirrors, wherein none of the fixed reflectors and none of the movable reflectors are oriented orthogonal to one another on a substrate when the movable reflectors are in a radiation reflecting position. In preferred embodiments, each of the fixed and movable reflectors is oriented parallel to or at a 70xc2x0 angle to, the remaining fixed and movable reflectors when the movable reflectors are in the radiation reflecting position. Most preferably, the fixed reflectors and the movable reflectors all are oriented on the substrate in parallel when the movable reflectors are in the radiation reflecting position. By providing these orientations of fixed and movable reflectors, add-drop optical switches may be fabricated on silicon substrates using wet etching along crystallographic planes. High performance add-drop optical switches thereby may be provided.
First embodiments of add-drop optical switches according to the present invention include a substrate, an ADD optical path on the substrate that receives radiation, an IN optical path on the substrate that receives optical radiation, an OUT optical path on the substrate that transmits optical radiation and a DROP optical path on the substrate that transmits optical radiation. As was described above, the optical radiation can include visible, ultraviolet, infrared and/or other forms of electromagnetic radiation. A plurality of fixed reflectors are fixedly coupled to the substrate. A plurality of movable reflectors are movably coupled to the substrate for movement to and away from a radiation reflecting position. The fixed reflectors and the movable reflectors are arranged on the substrate to selectively couple the IN optical path to the OUT optical path, to selectively couple the IN optical path to the DROP optical path, and to selectively couple the ADD optical to the OUT optical path. None of the fixed reflectors that are used to provide the above-described functionality are oriented orthogonal to one another on the substrate. Moreover, none of the movable reflectors that are used to provide the above-described functionality are oriented orthogonal to one another on the substrate when the movable reflectors are in the radiation reflecting position.
In preferred embodiments of the present invention, the substrate comprises monocrystalline silicon, and each of the fixed and movable reflectors is oriented parallel to or at a 70xc2x0 angle to the remaining fixed and movable reflectors when the movable reflectors are in the radiation reflecting position. In other preferred embodiments, all of the fixed and movable reflectors are oriented in parallel when the movable reflectors are in the radiation reflecting position. In preferred embodiments, the ADD, IN, OUT and DROP optical paths all are oriented on the substrate in parallel. In other preferred embodiments, the ADD, IN, OUT and DROP optical paths all are oriented on the substrate at a 45xc2x0 angle or at a 65xc2x0 angle relative to the fixed reflectors and the movable reflectors in the radiation reflecting position.
Other embodiments of the present invention orient the fixed reflectors and the movable reflectors on the substrate in parallel when the movable reflectors are in the radiation reflecting position and when the movable reflectors are away from the radiation reflecting position. In these embodiments, the movable reflectors may be slideably mounted on the substrate to move linearly to and away from the radiation reflecting position. In other embodiments, the movable reflectors may be oriented in parallel only when they are in the radiation reflecting position. They may be rotated flat against the substrate or rotated about an axis that is orthogonal to the substrate away from the radiation reflecting position.
In preferred embodiments of the present invention, the plurality of fixed reflectors comprise a first and a second fixed reflector, and the plurality of movable reflectors comprise a first and a second movable reflector. In other preferred embodiments, the IN optical path and the OUT optical path are collinear and the first and second movable reflectors are arranged between the IN optical path and the OUT optical path when the first and second movable reflectors are in the radiation reflecting position. The first and second movable reflectors preferably are arranged between the ADD optical path and the OUT optical path at a 45xc2x0 angle thereto. The first fixed reflector preferably is arranged on the substrate to reflect optical radiation from the first movable reflector to the DROP optical path and the second fixed reflector is arranged on the substrate to reflect optical radiation from the ADD optical path to the second movable reflector.
Other embodiments of the present invention can add a second ADD optical path, a second IN optical path, a second OUT optical path and a second DROP optical path on the substrate. A third fixed reflector and a third and a fourth movable reflector may be added. In preferred embodiments, the second IN optical path and the second OUT optical path are collinear, and the third and fourth movable reflectors are arranged between the IN optical path and the second OUT optical path when the third and fourth movable reflectors are in the radiation reflecting position. The first fixed reflector is between the DROP optical path and the first movable reflector and between the second ADD optical path and the fourth movable reflector. The second fixed reflector is between the ADD optical path and the second movable reflector. The third fixed reflector is between the second DROP optical path and the third movable reflector. Thus, these embodiments can allow the first fixed reflector to be shared by the first and second add-drop optical switches.
More generally, add-drop optical switches according to embodiments of the present invention can include a substrate, a plurality of ADD optical paths, a corresponding plurality of IN optical paths, a corresponding plurality of OUT optical paths, and a corresponding plurality of DROP optical paths on the substrate. An array of fixed reflectors is fixedly coupled to the substrate and an array of movable reflectors is movably coupled to the substrate for movement to and away from a radiation reflecting position. The fixed reflectors and the movable reflectors are arranged on the substrate to selectively couple a corresponding one of the IN optical paths to a corresponding one of the OUT optical paths, to selectively couple a corresponding one of the IN optical paths to a corresponding one of the DROP optical paths, and to selectively couple a corresponding one of the ADD optical paths to a corresponding one of the OUT optical paths. The fixed reflectors all are oriented on the substrate in parallel and the movable reflectors all are oriented on the substrate in parallel when the movable reflectors are in the radiation reflecting position. The plurality of ADD, IN, OUT and DROP optical paths also are oriented on the substrate in parallel, and at a 45xc2x0 angle relative to the fixed reflectors and the movable reflectors in the radiation reflecting position.
In preferred embodiments of multiple add-drop optical switches on a single substrate, the plurality of fixed reflectors comprise the corresponding plurality minus one of shared fixed reflectors and the plurality of movable reflectors comprise the corresponding plurality of first movable reflectors and the corresponding plurality of second movable reflectors. The corresponding first and second movable reflectors are arranged between the corresponding IN and OUT optical paths when the first and second movable reflectors are in the radiation reflecting position. The corresponding first and second movable reflectors are arranged between the corresponding IN and OUT optical paths, at a 45xc2x0 angle thereto, when the corresponding first and second movable reflectors are in the radiation reflecting position. The shared reflector(s) preferably are between a respective first and second reflector of a respective adjacent pair of collinear IN and OUT optical paths.
Add-drop optical switches may be fabricated, according to embodiments of the present invention, by etching a monocrystalline substrate only along crystallographic planes thereof, to form an array of fixed and movable reflectors. Parallel ADD, DROP, IN and OUT optical paths also may be fabricated on the monocrystalline substrate or on another substrate. The ADD, DROP, IN and OUT optical paths may be fabricated after or prior to etching the monocrystalline substrate. The etching step preferably comprises wet etching the monocrystalline substrate only along crystallographic planes thereof, to form the array of fixed and movable reflectors. For example, when the monocrystalline substrate is silicon, it may be etched only at 0xc2x0 and 70xc2x0 crystallographic planes.
Accordingly, add-drop optical switches may be provided that can be fabricated by wet etching along crystallographic planes of a silicon substrate. High performance add-drop optical switches thereby may be provided. Moreover, the parallel oriented reflectors can provide compact structures and array of add-drop optical switches that are amenable to low cost mass production.